From the Dota 2 website:
Today, we permanently banned 90,000 smurf accounts that have been active over the last few months. Smurf accounts are alternate accounts used by players to avoid playing at the correct MMR, to abandon games, to cheat, to grief, or to otherwise be toxic without consequence.
Additionally, we have traced every single one of these smurf accounts back to its main account. Going forward, a main account found associated with a smurf account could result in a wide range of punishments, from temporary adjustments to behavior scores to permanent account bans.
Smurfing is when you play Ranked ladder on an alt-account in a much lower skill bracket with the intention to curb stomp lesser-skilled players.
Itβs a very big issue in any competitive multiplayer game, especially direct competitors of DOTA 2 like League of Legends and SMITE. Valve may just be the first company to start actively banning smurfs.